Intertropical Springs and Rivera. 105 



having its bulb covered O'l of an inch Vith the same alluvium, stood 

 at 13 6°' 5. With and on the sand of the desert on the yergQ of the 

 inundation line, at the same hour, it stood 121°* 5. The tempera- 

 ture of the air at the time, five feet above the surface of the dark 

 alluvium, was 105°"6: the same height above that of the desert, it 

 was 103°' 5 ; sky unclouded, although the surface of the sands, dur- 

 ing the clear serene nights of I^gypt, is cooled considerably by radia- 

 tion, still a little below the surface, they retain a great portion of the 

 solar heat. In July, at sunrise, the surface of the sandy desert, on 

 the banks of the Nile at Thebes, latitude 25° 26' N., which, during 

 the heat of the day, indicated a temperature of 130*^, had cooled 

 down to 69°*5, while the thermometer, a foot below the surface, stood 

 at 83° : temperature of tlje air 75°. 



The temperature of the granite rocks in the beds of the Toombud- 

 di'a and the Kistnali, during the months of May and June, at 2 p.m., 

 I found from 118° to 120°: during the night they cooled down 

 usually to 83°. The temperature of the surface sands in these rivers 

 was slightly higher than that of the granite. 



The temperature of rivers whose supply, like those in South In- 

 dia, depends more on the periodical rains than on springs, is conse- 

 quently influenced by the temperature of the former. That of the 

 monsoon showers, which fell on the western coast near Mangalore, 

 during the months of May and June, varied from 73° to 79°, afford- 

 ing a mean of 7<3°. The rains falling on the elevated tableland of 

 the Ceded districts, from June to December, ranged from 7l°"5 to 

 97°* 5, giving the mean 75°*5. The mean general height of the 

 plain, between latitude 13° and 17° N., is 1300 feet above the sea 

 level. The temperature of the showers was invariably modified by 

 the conditions aftecting that of rain- water in extratropical countries, 

 namely, the elevation at which condensed, and the temperature of 

 the atmospheric strata through which the showers fell. 



The temperature of the Brahmapooti'a River at Gadya, in Assam, 

 was found in September by Mr Griffiths, to range from 63° to 70° ; 

 that of the air above the river, from 68° to 100° ; that of the Indus, 

 by Gerard, in March, near Attock, was 32°. 



Temperature of the Ocean on the Equator^ and between the Tro- 

 pics. — The influence of the trade-winds, cold currents from high la- 

 titudes, frequent showers, evaporation, &c. contribute to cool the air 

 and surface of the ocean at the equator. The extremes of the tem- 

 perature of the latter, at great distances from land, have been pretty 

 correctly stated by M. Arago, at 80°-8, and 84°- 2. On crossing the 

 line in the Atlantic Ocean (in longitude 20°*7 W.), I found the 

 temperature of the sea 84°* 5 ; air in the shade 87° : in the Indian 

 Ocean (longitude 58°-54 E.), 8r-5 ; air in the shade 82°-5. In the 

 same oceans, near the land, and in narrow seas, the range between 

 the extremes is much greater than 3°*4. In the Red Sea, from 

 the straits of Babelmandel to the tropic of Cancer, I found it, in the 



